1. Field of the Invention
The present invention relates generally to measuring, indicating, recording, and/or controlling apparatus of the electrical servo or self-balancing type, hereinafter referred to simply as servo apparatus. Specifically, the invention relates to the servo or rebalancing portion of such apparatus, wherein a servo motor adjusts the position of both a feedback element and a slaved display and/or control element as necessary to make that position representative of an electrical input signal or voltage. More specifically, the invention relates to the forms of such apparatus wherein travel limiting means are provided for effectively terminating the driving energization of the motor when the feedback and slaved elements have been moved to preset limit positions along their paths of travel.
2. Description of the Prior Art
In the known servo apparatus as described above, motor drive means are provided to compare the input voltage to a feedback voltage which represents the position of the motor-driven feedback element along its path of travel. The feedback element may well be the sliding contact of an adjustable feedback resistor. In response to such voltage comparison, the motor drive means feed upscale and downscale motor drive signals to the motor as required to cause the motor to keep the position of the feedback element repesentative of the input voltage.
In apparatus as just described, there is a tendency for the motor drive the feedback element past its acceptable upscale and downscale travel end points. Such points are defined as ones past which the feedback element should not or must not be moved. In those forms of said known apparatus wherein physical limit stops are provided on the feedback element travel path or its equivalent to define the acceptable travel end points, there is a tendency for the motor to continue in the stalled but energized condition to force the feedback element against such stops after that element has been driven to them. Such obviously undesirable tendencies to overdrive the feedback element occur when the input voltage moves outside of the measuring range for which the apparatus is arranged and calibrated.
The overtravel or overdrive tendencies just described are overcome in the above-noted known travel limiting forms of servo apparatus by the use of the noted means for terminating the motor energization when the feedback element has been moved to preset upscale and downscale travel limit positions. Such positions, which will be referred to hereinafter simply as limit positions, are usually established just short of the corresponding acceptable end points, or just short of the physical limit stops if present.
The establishment of the limit positions in the known apparatus is effected by so physically positioning or locating switching means that the latter are actuated when the driven feedback element reaches a point in its travel which is to be one of the limit positions. Such switch actuation interrupts or at least modifies the application of the motor drive signals to the motor, and hence terminates, or at least substantially reduces, the driving action of the motor which has produced the switch actuation. In this way, the motor is prevented from attempting to continue to move the feedback element in the same direction once a limit position has been reached. Consequently, the motor is prevented from attempting to drive the feedback element past the acceptable travel end points, and is prevented from continuing to force the feedback element against any physical limit stops which are present. The switching means in the known apparatus thus prevents the motor from driving or attempting to drive the feedback element out of its acceptable range of movement, and thus limits and restricts that range of movement to a desired range which lies between the limit positions.
In all of the forms of the known apparatus of which we are aware, the above noted switch actuation is effected by the movement of the feedback element, or of an element which is made to follow the feedback element, into a predetermined physical relationship with the switching means. The actuation is usually effected either mechanically, magnetically, optically, inductively, capacitively, or the like. An example of one form of such known apparatus is the apparatus of the Wild U.S. Pat. No. 2,633,558, wherein a member is caused to follow the feedback element, and wherein the switching means consists of switches which are mechanically acutated by the member when the latter reaches a position which corresponds to one of the limit positions. have performed satisfactorily in many applications, there has continued to be a need for a simpler form of such apparatus which would not be encumbered with the physically-actuated switching means of the known apparatus forms. Thus, there has been a need for a simplified travel limiting form of servo apparatus wherein the switching means would not require or utilize any physically movable parts, and wherein no particular physical position of the switching means relative to the moving portions of the servo apparatus would be required.